According to a new study by the University of Warwick, people who were bullied by siblings during childhood are up to three times more likely to suffer from illnesses such as schizophrenia and bipolar disorder in early adulthood.
Senior study author Professor Dieter Wolke from the University of Warwick's Department of Psychology, said: 'Bullying by siblings has been until recently widely ignored as a trauma that may lead to serious mental health problems such as psychotic disorder.
'Children spend substantial time with their siblings in the confinement of their family home and if bullied and excluded, this can lead to social defeat and self-blame and serious mental health disorder - as shown here for the first time.'
In the study, almost 3,600 12-year-old children from the Avon Longitudinal Study of Parents and Children were asked to fill out a detailed questionnaire on sibling bullying.
From that questionnaire, 664 children were victims of sibling bullying, 486 were the bully to their siblings and 771 were bully-victims (children who become both the victim and bully their siblings).
Those same 3,600 children were then tasked with completing a standardised clinical examination assessing psychotic symptoms when they reached 18-years-old.
From the total 3,600 children in the study, 55 of them had developed a psychotic disorder by the age of 18.
The researchers found that the more frequently children are involved in sibling bullying - either as bully, victim, or both - the more likely they are to develop a psychotic disorder.
The children most at risk are victims of sibling bullying, and those who both become victims and bully their siblings (bully-victims), according to the study.
The study also found that children who are bullied both at home and at school are even more likely to suffer in later life as they have four times the chance to develop psychotic disorders than those not involved in bullying at all.